1. Field of the Invention
The present invention relates to a motor driving apparatus, a brushless motor, and a method for driving the brushless motor.
2. Related Art
In a brushless motor widely used in electrical apparatuses as a motor, the position of a rotor which includes a permanent magnet is detected by a plurality of position detectors (e.g., Hall elements) which are arranged at regular intervals near the rotational orbit of the rotor, and the rotor is rotatively driven by switching the energization statuses of a plurality of driving coil windings in synchronism with changes in detected signals between the on and off states. For example, a three-phase brushless motor is generally configured to include three coil windings and generate a rotational torque by repeating switching of the energization statuses of two of the three coil windings.
As a motor driving apparatus which switches the energization statuses (voltage application statuses) of three driving coils of a three-phase brushless motor, there is proposed one including a power supply, a switching circuit which is provided between power supply lines and connected to driving coils, a PWM control section which switches the mode of driving between synchronous rectification PWM driving and one-way PWM driving using a decelerating signal for speed control, a pulse width modulating section, a position detecting section, and an energization switching section (see, e.g., Japanese Patent Laid-Open No. 2002-272162).
In the switching circuit, series circuits of an upper transistor and a lower transistor are provided as switching elements corresponding to three phases. A diode for protection against counter electromotive force is connected in parallel with each transistor. One ends of three driving coils are respectively connected to connection points between the upper transistors and the lower transistors, and the other ends are commonly connected. Drains of the three upper transistors are commonly connected, and sources of the three lower transistors are commonly connected.
The position detecting section detects the positional relationship between a rotor magnet and the driving coils. The energization switching section outputs an energization switching signal to the driving coils on the basis of an output from the position detecting section. The pulse width modulating section outputs a frequency signal with a pulse width corresponding to a torque command signal for controlling a torque generated by a brushless motor. The PWM control section performs on/off control of the transistors of the switching circuit on the basis of an energization switching signal and a signal output from the pulse width modulating section.
The operation of the aforementioned motor driving apparatus will be described. The three driving coils will be referred to as driving coils A, B, and C. The upper transistor, to which one end of the driving coil A is connected, will be referred to as a transistor TAu while the lower transistor, to which the one end of the driving coil A is connected, will be referred to as a transistor TAd. Similarly, the upper and lower transistors, to which one end of the driving coil B is connected, will be referred to as transistors TBu and TBd, respectively, while the upper and lower transistors, to which one end of the driving coil C is connected, will be referred to as transistors TCu and TCd, respectively.
When the driving coils A and B are energized, the transistor TAu is turned on, the transistor TAd is turned off, and the transistors TBu and TBd perform on-off operation in synchronous rectification PWM driving, except during a torque command-based decelerating period such as when in a steady state. PWM stands for Pulse Width Modulation and is a method for controlling power by changing the duty ratio (the ratio of the on time) of pulses for driving a motor. To cause both the upper and lower transistors to perform PWM operation is referred to as synchronous rectification PWM driving. When the transistor TBu is on, the transistor TBd is off. On the other hand, when the transistor TBu is off, the transistor TBd is on.
During a torque command-based decelerating period, turning on or off of the transistor TBd does not cause the transistor TBu to be turned on or off. Accordingly, when the transistor TBd is off, a regenerative current flows through the diode connected in parallel with the transistor TBu. This is referred to as one-way PWM driving. In the case of synchronous rectification PWM driving, during a decelerating period, the pulse width (duty ratio) for PWM driving decreases. A current flowing backward from the driving coils to a power supply may increase a power supply voltage and cause device breakdown. Switching of the mode of driving to one-way PWM driving in accordance with a decelerating signal for speed control makes the driving efficiency lower than that in synchronous rectification PWM driving but makes it possible to prevent device breakdown.
As described above, except during a torque command-based decelerating period such as when in a steady state, the motor driving apparatus prevents a current from flowing through the diodes for protection against counter electromotive force and can implement low-loss driving. During a decelerating period, the motor driving apparatus prevents a current from flowing backward to the power supply and prevents device breakdown.
A rise in power supply voltage caused by a current flowing backward to a power supply varies depending on the impedance of the power supply. A rise in voltage decreases with a decrease in power supply impedance. If another circuit is connected to the power supply, and the power supply impedance apparently becomes low, a rise in voltage becomes small. A rise in voltage also becomes small depending on a decrease in the speed or the number of revolutions of a motor and may not lead to device breakdown. However, the aforementioned motor driving apparatus switches between synchronous rectification PWM driving and one-way PWM driving using a decelerating signal for speed control. Accordingly, whenever a deceleration command is issued, the motor driving apparatus switches to one-way PWM driving even if a rise in voltage stops short of leading to device breakdown, and the driving efficiency decreases.